The present invention relates to optical recording methods and more particularly to a technique based on land/groove recording and suitable for performing high-density optical recording in which the track width is smaller than the optical spot diameter.
A conventional method is disclosed in, for example, JP-A-59-191156. In the prior art, a laser beam generated from a laser diode carried on an optical head is formed into a collimated beam by means of a collimating lens, and the collimated beam passing through a beam splitter is focused by an objective lens so as to be converged into an optical spot on a magneto-optical recording medium. The position of the optical spot on the magneto-optical recording medium is controlled by moving the lens or the optical head by means of an optical spot scanning control means. Reflected light from the magneto-optical recording medium is guided to a photodetector through a beam splitter. A readout signal from the photodetector is processed by a reproduction circuit so as to be converted into reproduced data. Control of overall reproduction is carried out by a controller.
JP-A-6-176404 describes a technique for performing high-density (narrow track) recording.
A recording medium disclosed in JP-A-6-176404 is illustrated, in plan view form, in FIG. 5. Grooves 501 and lands 502 are formed on a substrate, information recording areas are formed in association with both the groove and the land, and prepits 504 are disposed on an extension line 503 of the boundary line between a groove 501 and a land 502. Prepits 504 are positioned each groove on only one side relative to the center line of each groove. With this construction, recording information is recorded on both the groove 501 and the land 502, address information representative of the recording areas are recorded in the form of prepits 504, and one prepit is used in common to a pair of adjacent groove 501 and land 502 to provide address information therefor.
When the technique as above is applied to, for example, a phase change recording medium or a magneto-optical recording medium, interference of information (crosstalk) between adjacent grooves 501 or lands 502 due to the optical interference effect within an optical spot 505 can be prevented, thereby permitting narrowing of track. On the other hand, in the prepit area free from the optical interference effect, the address information can be common to the paired groove and land and the effective track pitch can be increased to reduce crosstalk.
In the example of JP-A-6-176404, however, the disposition of the prepit area is offset on one side of the center line of the groove and an offset tracking error signal is delivered out of the prepit area, with the result that when an optical spot is caused to track a groove or a land, a tracking error (tracking offset) increases, making it difficult to perform high-density recording in which the track pitch is narrowed.
An object of the present invention is to provide a technique capable of suppressing the tracking offset to a value which is sufficiently low for practical use and permitting efficient disposition of address information even when recording is effected on both the groove and the land.
To accomplish the above object, solutions of the invention are adopted as below.
More particularly, in an optical recording medium having substantially concentric grooves and lands formed on a circular substrate and information recording areas formed in association with both the groove and the land, prepits are disposed on a virtual extension line of the boundary between a groove and a land, the disposition of the prepits satisfying all of the following four conditions:
(i) Prepits are located on both sides of an extension of the center line of one groove;
(ii) Prepits are located on both sides of an extension of the center line of one land;
(iii) Prepits are not located on both sides of any specific position of the center line of one groove; and
(iv) Prepits are not located on both sides of any specific position of the center line of one land.
With this construction, disposition of prepits is not offset on either one side of a virtual extension of the center line of the groove or the land, so that an offset tracking error signal is not delivered out of the prepit area, making the tracking offset hardly occur. Further, since prepits do not exist on both sides of or symmetrically to a position on an extension of the center line of the groove or the land, interference of prepit information between adjacent tracks do not take place within a reproduction spot. Accordingly, recording can be performed on both the groove and the land and addresses can be reproduced without crosstalk to permit high-density narrow track recording.
Preferably, prepits are disposed alternately at a period which is even times a channel bit length on both sides of a virtual extension of the center line of the groove.
Thus, the prepits are uniformly disposed on both sides of a virtual extension of the center line of the groove or the land, making the tracking offset more hardly occur.
Further, the groove and the prepit have the same depth which is 70 nm or less. More preferably, the depth is 40 nm or more and 60 nm or less.
Through this, crosstalk between the groove and the land can duly be canceled and an excellent tracking servo signal can be obtained, thus making injection and production of a medium easy. With the groove depth being in excess of 70 nm, injection of the groove is difficult to achieve. With the groove depth being about 50 nm, tracking servo is maximized and substantially the same effect can be obtained at a groove depth which is 10 nm around 50 nm.
Preferably, the groove and the land have substantially the same width which is in the range of from 0.3 xcexcm to 0.75 xcexcm.
Through this, excellent tracking can be compatible with high-density recording. If the groove and the land has a width which is not greater than 0.3 xcexcm, two sets of groove and land are concurrently within a single optical spot and any excellent tracking signal cannot be obtained. With the width of the groove and the land being in excess of 0.75 xcexcm, practical high-density recording cannot be permitted.
The minimal diameter of a prepit is made to be smaller than the width of each of the groove and land. More preferably, the diameter falls within the range of from 0.25 xcexcm to 0.55 xcexcm.
Through this, an excellent prepit signal can be obtained without crosstalk. If the diameter is not greater than 0.25 xcexcm, power of the prepit signal decreases extremely and with the diameter being in excess of 0.55 xcexcm, crosstalk takes place.
When an optical recording medium is used in which grooves and lands are formed on a substrate, information recording areas are formed in association with both the groove and the land, any groove is not formed but flat address areas are discretely formed in the information recording area, and first and second address pits are disposed in the address area on an extension of the boundary between the groove and the land, the first and second address pits being disposed to satisfy such requirements that the first and second address pits are disposed alternately on both sides of an extension of the center line of one groove, that the first and second address pits are disposed alternately on both sides of an extension of the center line of one land, that address pits do not exist on both sides of a position on an extension of the center line of the groove and that address pits do not exist on both sides of a position on an extension of the center line of the land, an optical spot is irradiated on the optical recording medium, a reflected beam from the optical recording medium is detected, an address pit is detected from the detected reflected beam to form an address pit readout signal, an address is detected on the basis of the address pit readout signal, an amplitude of a first readout signal obtained from the first address pit of the address pit readout signal is sampled and held, an amplitude of a second readout signal obtained from the second address pit of the address pit readout signal is sampled and held, the amplitudes of the first and second readout signals are compared together, an offset signal is formed on the basis of a result of comparison, and the irradiation position of the optical spot is controlled on the basis of the offset signal.
Through this, the tracking offset can be suppressed sufficiently for practical use and address information can be obtained.
When tracking is carried out by sequentially obtaining tracking servo signals through the use of a diffracted beam obtained from a groove and correcting an offset of a tracking servo signal with an offset signal, stabler tracking can be ensured. More particularly, upon detection of a reflected beam from the optical recording medium, a tracking servo signal is formed by detecting a light beam diffracted by a groove by means of a plurality of photodetectors, comparing diffracted beams detected by the plurality of photodetectors and detecting the relative positional relation between the groove and the optical spot, the tracking servo signal is corrected with an offset signal, and the irradiation position of the optical spot is controlled on the basis of the corrected tracking servo signal.
Further, an optical recording/reproducing apparatus may be constructed which uses a similar optical recording medium, comprises a light beam source, a beam focusing means for focusing and irradiating a light beam generated by the light beam source on the optical recording medium, photodetecter detecting a reflected beam of the light beam irradiated by the beam focusing means, a reproduction circuit reproducing information by using a signal from the photodetector, and a scanner moving the position of an optical spot irradiated by the beam focusing means to a desired position on the optical recording medium, and further comprises means for detecting an address on the basis of a readout signal from a prepit, a low-pass filter for detecting an amplitude of a low frequency component of the readout signal from the prepit, and a circuit performing lock-in detection of a passed signal of the low-pass filter, whereby the position of the optical spot is controlled on the basis of the detected signal.
With this construction, the tracking offset can be suppressed to a smaller value.
According to an aspect of the present invention, as shown in for example FIG. 1, prepits are disposed on both sides of a virtual extension of the center line of the groove or-the land in staggered relation. Accordingly, offset can be decreased to make the tracking offset hardly occur and prepits do not exist on both sides of a position on the extension of the center line of the groove or the land, with the result that interference of prepit information between adjacent tracks can be prevented within a reproduction spot and high-density narrow track recording can be ensured.
Further, even if a tracking offset takes place as shown in FIG. 3, signal amplitudes of prepits on both sides are compared to perform accurate detection of the tracking offset amount. Accordingly, by feedback-controlling the information to the scanner, the tracking offset can be suppressed.